1. Field of the Invention
The present invention relates to a photomask for use in exposure in a projection type exposure system, and more specifically to a photomask for exposure that is suitable for exposing a material to be exposed, such as a semiconductor substrate, that has a step.2.
2. Description of Related Art
In recent years, an increase in the level of integration and a minimizing of the feature sizes in semiconductor devices as typified by DRAMs (dynamic random-access memories) has made the circuit pattern line width formed on a semiconductor substrate extremely small. Accompanying this, there is a need to transfer even smaller patterns using the lithography process that forms circuit patterns onto a semiconductor substrate.
In the currently used lithography process, a reducing projection-type exposure system (such as a stepper) is used to burn the circuit pattern of a photomask, using ultraviolet light into the photoresist that has been applied to the semiconductor substrate, so as to form the pattern.
To form the pattern, it is ideal to cause the surface of the substrate to coincide with the image-forming plane of the projection lens.
Because of the steps that occur to form elements and because the substrate itself is not flat, there occurs a shift between the two.
To form a pattern even if there is somewhat of a shift from the ideal image-forming plane, some depth of focus (range in the light axis direction over which the pattern can be formed) is necessary, and the achievement of this depth of focus is important, as is high resolution.
In general, the resolution R and depth of focus DOF in a lithography process that uses a reducing exposure method is given by the following equations, which are known as the Raleigh equations. EQU R=K1.multidot..lambda.1/NA (1) EQU DOF=K2.multidot..lambda.1/(NA).sup.2 (2)
In the above relationships, .lambda. is the exposure wavelength in nm, NA is the numerical aperture of the lens, and K1 and K2 are process coefficients that depend on the resist process.
As can be seen from equation (1), the limiting resolution can be improved by making .lambda. small and by making NA large. However, from equation (2), it can be seen that a short wavelength and a large NA result in a reduced depth of focus, DOF. At present, there is a sharp decrease in the depth of focus accompanying an improvement in resolution, thus making it difficult to achieve the required depth of focus.
As can be seen from equation (2), the reduction in the depth of focus is more gradual for a shortening of wavelength than it is for an increase in NA. For this reason, the wavelength has been made progressively smaller, there having been a shift from the g line (1=436 nm) to the i line (1=365 nm) of a mercury lamp, and further to the use of a KrF excimer laser (1=248 nm) as the light source for exposure.
Additionally, by making adjustments in the optics itself, it is possible to accommodate the above-described shrinking of features in the pattern, this overall approach being referred to as super-resolution technology.
Typical aspects of super-resolving technology can be classified into three types, according to where the adjustments are made, these being the approach of changing the light source shape in the projection optics (the deformed illumination method), the approach of changing the pupil plane of the projection lens (the pupil filter method), and the approach of changing the photomask used for exposure (phase shift method, halftone phase shift method, or the like).
The effect of these types of super-resolving technology depends upon the type of pattern on the exposure photomask.
In the halftone phase shift method (hereinafter referred to as the halftone method), instead of using a complete light-blocking film in the light-blocking section on the photomask, a translucent film which rotates the polarization of incident light by 180 degrees and partially passes the light is used, as disclosed in the Japanese Unexamined Patent Publication No. 8-31711, for example.
FIG. 12 shows the exposure photomask used in the past with the halftone method. As shown in FIG. 12(a) and (b), an exposure photomask of the past was a two-layer structure formed from a glass substrate 101 that serves as the mask substrate, and translucent film 102 which is attached to this glass substrate 101.
According to this halftone method, there is emphasis of high-order diffraction components in the mask, resulting in an improvement in the contrast of the light intensity on the wafer over a wide depth of focus.
The halftone method is particularly effective in improving the depth of focus for an isolated transmission pattern (a hole pattern for the case of a positive pattern). For example, in Y. Iwabuchi, et al, Jpn. J. Apply. Phys. 32 (1993) 5900, there is a description of the use of KrF excimer lithography to form an isolated 0.26 .mu.m hole pattern. With NA=0.42 and s=0.5 (where s is the coherency factor), using a conventional mask, the depth of focus for a 0.26 .mu.m hole pattern is 0.6 .mu.m.
In contrast to this, by exposing using a halftone photomask having a transmissivity of 4%, the focal point is broadened to 1.2 .mu.m.
Because the halftone method makes the fabrication of the mask relatively easy, it is currently being applied to mass production, chiefly using the iline lithography. Additionally, it has been reported that to maximize the effect of improving the focal point, it is desirable to make the phase shift be 180.+-.5 degrees and to make the transmissivity be 5 to 10%.
However, in recent semiconductor devices, accompanying the increase in the level of integration, the cross-sectional structure has become extremely complex.
For this reason, the steps formed in the semiconductor substrate are of a size that cannot be compensated by using the above-described halftone method. Consider, for example, the case of applying the above-described exposure wavelength, optics conditions, and a halftone mask to form a 0.26 .mu.m hole pattern in a substrate having a step of 0.6 .mu.m. Because the focal point is 1.2 .mu.m, a maximum up/down focusing margin of 0.6 .mu.m is obtained. However, with exposure done with the focus position at the up/down center point position of the 0.6 .mu.m step, the margin is 0.3 .mu.m at the top part of the step and 0.3 .mu.m at the bottom part of the step.
That is, considering the step the focal point is substantially limited to 0.6 .mu.m.
As described above, the achievement of a substantially wide focal point with regard to a semiconductor substrate having a step as well represents a problem in terms of device fabrication.
In view of the above-described drawbacks in the prior art, an object of the present invention is to provide an exposure photomask capable of forming a desired pattern over the entire region of even a material to be exposed which has a step.